CHERI is insufficient for unsafe Rust to be fully safe, and I suspect Fil-C plus C's restrict is insufficient as well (if restrict isn't treated as a no-op, of course).

For unsafe Rust code to be sound, it has to uphold the invariant that no location pointed to by an &mut is modified through a pointer not derived from that &mut. However, there's no such restriction on *mut pointers ("raw pointers"), and it is in many cases perfectly sound (but unsafe, of course) to cast from *mut to &mut, or to directly mutate through a *mut. Without maintaining global information about what references are live, it sounds really, really hard to guarantee this.